Multifrequency operating radome

ABSTRACT

A radome designed to transmit two electro-magnetic waves of different frequencies. The dielectric wall has a thickness such that it is transparent vis-a-vis the first wave of frequency F1. A network of continuous wires integral with the wall renders the latter transparent vis-a-vis the second wave of frequency F2 lower than F1. Discontinuous metal elements integral likewise with the wall are arranged in accordance with a second network in order to eliminate the grating lobes due to the presence of the network of continuous wires.

United States Patent 3,864,690 Pierrot Feb. 4, 1975 [54] MULTIFREQUENCYOPERATING RADOME 3,560,986 2/1971 Hoots etal. 343/872 Inventor: RobertPierrot Paris, France 3,633,206 1/1972 McMillan 343/872 [73] Assignee:Thomson-CSF, Paris, France Primary Examiner-Eli Lieberman [22] Filed:Nov. 1 1973 Attorney, Agent, or Firm-Edwin E. Greigg [21] Appl. No.:411,911 [57] ABSTRACT A radome designed to transmit two electro-magnetic[30] Fore'gn Apphcatlon Prmmy Data waves of different frequencies. Thedielectric wall has Nov. 3, 1972 France 72.38959 3 thickne s u h that itis transparent vis-a-vis the first wave of frequency F 2% 2g A networkof continuous wires integral with the wall n. q renders the lattertransparent vis a vis the Second [58] Field of Search 343/705, 708, 872,909, wave of frequency F2 lower than F1 343 911 R Discontinuous metalelements integral likewise with the wall are arranged in accordance witha second 5 6] References Cited UNITED STATES PATENTS network in order toeliminate the grating lobes due to 2,978,702 4/1961 Pakan 343/909 thepresence of thfi network contmuous wlres. 3,148,370 9/1964 Bowman343/909 12 Claims, 6 Drawing Figures "E1 fi E] [I IIIDEIUGEIUEIDUEIEIDUEIEIU III El E] [I] l] E] s 'i: m E] 8 E] ElDUIIIEIEIDEIDUEIEJUEIDE] [:1 r3 El 7 El E] El 1] E] El E] [I E]EJDUEIEIEIDEIEIDEIEIEJEJE] El E] El El E1 El El )m wwfl PATYENTEDFEB4:975

SHEEIEOFZ 5 \Id/\ Damn BUUUU U U U m. U m U U U m U m UUUUUUUUUUUUUUU Um U U U m m U m U U U mummmummmnmmmmmm U m m U U n u U n i k MMULTIFREQUENCY OPERATING RADOME This invention relates to radomes usedto protect radio equipment in supersonic aircraft.

The only location available in aircraft of this kind, for the housing ofthe equipment in a radome, is the nose, which due to the aerodynamic andmechanical stresses imposed upon it, has a conical or ogival form. Thesestresses also make it necessary to utilise a monolithic or homogeneousdielectric wall as for example a fibreglass weave impregnated withresin, or an epoxy glass. The electrical characteristics of the wall aredetermined by the wave passing through it. The wall, in all cases, has aminimum thickness corresponding to half a wavelength in the dielectric.

The utilisation on board such aircraft, of several electromagneticsystems operating at widely different frequencies, is generally out ofthe question. The electrical characteristics of the wall being matchedto a first wave, are generally not so in respect of another since, thecoefficient of reflection of the wall relative to such other wave, isthen too high. For a second wave of frequency lower for example thanthat of the first, the dielectric wall thus appears as either too thinin relation to the half wavelength or too thick to be considered as anegligible skin.

DESCRIPTION OF THE PRIOR ART It is known, in this context, toincorporate into the dielectric wall networks of wires which areinductive when the polarisation is parallel to them. The overallassembly then behaves as a band-pass filter whose elements are designedso that they present either a very wide pass-band or responses at twodifferent frequencies.

These networks of conductor wires in effect constitute a kind of gridthrough which the electromagnetic waves have to pass. Each free spacebetween the conductors constitutes a radiating aperture. Phasedistribution in the radiating apertures is not uniform since the radomeis not flat and the result is accordingly a distortion in the radiationpatterns of the antennas located inside the radome. This distortion istranslated by the appearance of secondary lobes or grating lobes.

The object of the present invention, is precisely that of providing aradome which does not produce such distortion.

SUMMARY OF THE INVENTION In accordance with the invention, the radomecomprises a monolithic dielectric wall whose thickness is such that itis transparent for at least a first wave of frequency F1, a firstnetwork of continuous wires integral with the wall, designed in such afashion that it acts in concert with said wall as an assembly resonatingat a second frequency F2 lower than the frequency F1, and an assembly ofdiscontinuous metal elements, likewise integral with the wall anddistributed in accordance with the lines of a second network interleavedwith said first network.

The invention and its application will be better understood from areading of the following description and by reference to the figures inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a graph showing thetransmission factor as a function of frequency, for a wall with orwithout metal elements;

FIGS. 2 and 3 illustrate equivalent impedance networks for a radomecomprising a network of continuous wires, at two frequencies F1 and F2respectively;

FIGS. 4 and 5 represent the equivalent impedance networks of a radomecomprising networks of continuous wires and discontinuous elements, fortwo respective frequencies F2 and F1;

FIG. 6 illustrates metal networks printed on a dielectric substrate.

FIG. 1 illustrates a graph showing the value of the transmission factorT of the wall of a radome as a function of the frequency F of a wavepassing through it, and also for a given wall thickness, a givendielectric constant and a given angle of incidence.

The graph 1 illustrates the case of a radome which does not incorporateany metal elements. At the low frequencies, the thickness appears smallcompared with the wavelength, and the radome behaves as a very thin skinwhose attenuative effect is negligible. The transmission factor is thenequal to 1. As the frequency rises, the thickness becomes progressivelygreater in relation to the wavelength and the transmission factorreduces. However, the factor returns to a value close to 1 whenever thethickness of the dielectric is equal to a multiple of the halfwavelength in the dielectric, that is to say to a given thickness e, atthe frequencies Fl, 2F1, 3F 1.. where F1 is the frequency at which thewavelength in the dielectric is equal to twice the thickness e of thewall. It should be pointed out, too, that the pass-band at thefrequencies 2Fl, 3P1, becomes progressivelynarrower.

The graph 2 illustrates the case of a radome comprising at least onenetwork of wires. For frequencies other than those which are multiplesof F1, the dielectric wall constitutes a capacitive or inductivebarrier. Each wire network, integral with the wall, acts as barriers, ofinductive or capacitive kind. The combination of the wall and the wirenetworks, constitutes therefore an assembly which resonates at aselected frequency F2 at which frequency, furthermore, the transmissionfactor of the wall becomes close to unity. The radome is thustransparent to at least two waves, of frequencies F1 and F2. Bycontrast, it becomes opaque to low frequencies, the metal barriersconstituting a Faraday cage. At higher frequencies, the two graphs 1 and2 merge together.

FIG. 2 illustrates an equivalent impedance network corresponding to thefrequency F2, for a radome equipped with a network of continuous wires.The dielectric wall and the wires are equivalent from the radio point ofview, respectively to a capacitance Cl and and inductance L1 in parallelwith a transmission line 3 loaded at one end by the electromagneticdevice operating at frequency F2, and at the other end by the spaceoutside the radome. By satisfying the condition LlCltuZ at the frequencyF2 m2/21r, and neglecting ohmic losses, the overcall impedance placed inparallel with the transmission line 3, is rendered infinite and the wavetransmitted is unaffected. The transmission factor, in reality, ispractically equal to l, the wall matching condition being achieved atthe frequency F2.

In order to effect simple resolution of the delicate problem of theorientation of the wires in relation to the polarisation, the inductiveelement is constituted by two networks of wires at right-angles to oneanother, this rendering it isotropic and making it possible to installit in the radome without any problem associated with orientation of thewires.

FIG. 3 illustrates the equivalent of the same radome, at the frequencyF1. The value of the inductance L2 placed in parallel by the metalnetwork, is much higher than it is at the frequency F2. Its effect issmall and can be compensated by a slight increase in the thickness ofthe dielectric wall whose equivalent impedance is then that of a smallcapacitance C2 shunted across the input terminals of the four-terminaldevice Q representing the wall which is matched at frequency Fll only.By satisfying the condition L2C2wl l at the frequency Fl (Oi/27f, thewall matching condition is achieved.

DESCRIPTION OF THE PREFERED EMBODIMENTS Instead of simply utilising anetwork of continuous wires, the radome in accordance with the inventioncomprises a network of continuous wires and a network of discontinuousmetal elements (networks 7 and 8 shown in FIG. 6). This variantembodiment has the advantage over the network of continuous wires, thatit does not introduce any secondary lobes or so-called grating lobes, atthe frequency F1, into the radiation pattern of the antenna protected bythe radome.

The discontinuous metal elements can take various forms. They arearranged in lines to produce a network of parallel lines or rather twomutually perpendicular networks like the network or networks ofcontinuous wires. The aligned sets of discontinuous elements areinterposed between the continuous wires. The pitch of the network of thediscontinuous elements can be the same as that of the continuous wires(FIG. 6) or may be larger or smaller, depending upon the desired effect.The metal elements are constituted either by segments ofwire, or bysegments of metal tape of given width and thickness. In the case shownin FIG. 6, the network 8 is constituted by metal squares whose sideshave a length of around l\/8 for the highest frequency, the pitch of thenetwork being equal to )t/2. In this case, the radome comprisescontinuous wires and rows of discontinuous elements, in alternation.

FIG. 4- illustrates the equivalent impedance network of the network andwall assembly, at the frequency F2. The networks are equivalent to aninductance L3 and a capacitance C3 in parallel with a capacitance Cllrepresenting the wall. The matching conditons L3 (C1 C3)m2 l is achievedwith an inductance L3 smaller than the inductance L1 in the case ofanetwork of continuous wires only. The inductance L3 is made smaller byreducing the pitch interval of the network of wires. It is possible inthis fashion to reduce the pitch sufficiently to prevent the developmentof grating lobes at the frequency Fl.

FIG. 5 illustrates the equivalent impedance networks the frequency Fl.The network is again equivalent to an inductance L4 of high value and toa capacitance C4 of low value, shunted across the terminals of thematched fourterminal device representing the wall. By satisfying thecondition L4C4wl l, the assembly is transmission matched.

The utilisation of discontinuous metal elements, introduces a capacitiveimpedance at the wall. One advantage of the invention resides in thefact that the dielectric wall can be transmission matched, even in thecase where the wall appears as an inductive barrier at the frequency F2.It is then necessary for the network of discontinuous elements to becapable of presenting 5 an adequate capacitive impedance.

The design ofa radome in accordance with the inven tion can be effectedin various ways. The networks of continuous and discontinuous metalelements are first of all etched, printed, stuck or deposited byvaporisation, on a dielectric substrate (for example a substrate ofpolyethyleneterephthalate) which will subsequently form an integral partof the wall of the radome.

The wall itself is made of ceramic or of some refractory material oragain of resin-impregnated fibreglass.

In the case of fibreglass, the radome is manufactured by arranging on amould a tissue of fibreglass which is stiched and then impregnated withresin. Another method consists in weaving or winding the glass fibresdirectly on to the mould, this avoiding the need for stiching.Generally, several layers of impregnated tissue or weave are required inorder to achieve the desired thickness.

The introduction of the metallised substrate is effected prior to orduring the building of the wall. The substrate is then placed on themould before the winding on or before the introduction of the glasstissue or weave. It can then be arranged between two layers of saidtissue.

Another method consists in producing a metallised substrate which simplycarries the networks of discontinuous metal elements. Independently thenetwork of continuous wires is incorporated in the radome wall duringthe building up of same. Then the metallised substrate and the radomeare joined. In the case of a radome which already contains a network ofcontinuous wires, a substrate metallised with a network of discontinuouselements and produced in the same mould as that used for the radome, isstuck to the interior of the latter.

In all the cases, the introduction of the substrate can be effectedafter the building of the wall. The metallised substrate is stuck to theinterior of the radome. Its assembly is a relatively simple matter ifthe ogival shape is not too different from that of a cone, and this isgenerally the case.

What is claimed is:

I. A multifrequency operating radome comprising:

a monolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics;

a first network of continuous wires, said network being integral withsaid wall, for constituting with said wall an assembly tuned for asecond wave to a second frequency lower than the first frequency, saidnetwork being the origin of grating lobes at the first frequency, and,

a second network of discontinuous metal elements, said second networkbeing likewise integral with said wall, for compensating for saidgrating lobes; and

wherein the network of continuous wires and the network of discontinuouselements are printed on a dielectric substrate, said substratesubsequently being stuck to the internal face of the dielectric wall.

2. A radome as claimed in claim I, wherein each of the discontinuousmetal elements is constituted by a segment of wire of short length,compared with the shortest operating wavelength.

3. A radome as claimed in claim 1, wherein the network of discontinuouselements and the network of continuous wires have the same pitch.

4. A multifrequency operating radome comprising:

a monolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics;

a first network of continuous 'wires, said network being integral withsaid wall, for constituting with said wall an assembly tuned for asecond wave to a second frequency lower than the first frequency, saidnetwork being the origin of grating lobes at the first frequency, and,

a second network of discontinuous metal elements, said second networkbeing likewise integral with said wall, for compensating for saidgrating lobes; and

wherein the network of continuous wires and the network of discontinuouselements, are printed on a dielectric substrate, said substratesubsequently being incorporated into the thickness of the dielectricwall.

5. A multifrequency operating radome comprising:

a monolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics;

a first network of continuous wires, said network being integral withsaid wall, for constituting with said wall an assembly tuned for asecond wave to a second frequency lower than the first frequency, saidnetwork being the origin of grating lobes at the first frequency, and,

a second network of discontinuous metal elements, said second networkbeing likewise integral with said wall, for compensating for saidgrating lobes; and

wherein said dielectric wall comprises at least one tissue of glassfibres, the network of continuous wires being incorporated into thetissue during the weaving stage, and the network of discontinuous metalelements being printed on a dielectric substrate which is subsequentlystuck to the internal face of the dielectric wall.

6. A multifrequency operating radome comprising:

a monolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics;

a first network of continuous wires, said network being integral withsaid wall, for constituting with said wall an assembly tuned for asecond wave to a second frequency lower than the first frequency, saidnetwork being the origin of grating lobes at the first frequency, and, r

a second network of discontinuous metal elements, said second networkbeing likewise integral with said wall, for compensating for saidgrating lobes; and

wherein said dielectric wall comprises at least one tissue of glassfibres, the network of continuous wires being incorporated into thetissue during the weaving of the same, and the network of discontinuousmetal elements being printed on a dielectric substrate which issubsequently incorporated into the thickness of the dielectric wall.

7. A radome as claimed in claim 4, wherein each of the discontinuousmetal elements is constituted by a segment of wire of short length,compared with the shortest operating wavelength.

8. A radome as claimed in claim 4, wherein the network of discontinuouselements and the network of continuous wires have the same pitch.

9. A radome as-claimed in claim 5, wherein each of continuous wires havethe same pitch.

1. A multifrequency operating radome comprising: a monolithic dielectricwall for transmitting a first wave of a first frequency and itsharmonics; a first network of continuous wires, said network beingintegral with said wall, for constituting with said wall an assemblytuned for a second wave to a second frequency lower than the firstfrequency, said network being the origin of grating lobes at the firstfrequency, and, a second network of discontinuous metal elements, saidsecond network being likewise integral with said wall, for compensatingfor said grating lobes; and wherein the network of continuous wires andthe network of discontinuous elements are printed on a dielectricsubstrate, said substrate subsequently being stuck to the internal faceof the dielectric wall.
 2. A radome as claimed in claim 1, wherein eachof the discontinuous metal elements is constituted by a segment of wireof short length, compared with the shortest operating wavelength.
 3. Aradome as claimed in claim 1, wherein the network of discontinuouselements and the network of continuous wires have the same pitch.
 4. Amultifrequency operating radome comprising: a monolithic dielectric wallfor transmitting a first wave of a first frequency and its harmonics; afirst network of continuous wires, said network being integral with saidwall, for constituting with said wall an assembly tuned for a secondwave to a second frequency lower than the first frequency, said networkbeing the origin of grating lobes at the first frequency, and, a secondnetwork of discontinuous metal elements, said second network beinglikewise integral with said wall, for compensating for said gratinglobes; and wherein the network of continuous wires and the network ofdiscontinuous elements, are printed on a dielectric substrate, saidsubstrate subsequently being incorporated into the thickness of thedielectric wall.
 5. A multifrequency operating radome comprising: amonolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics; a first network of continuous wires, saidnetwork being integral with said wall, for constituting with said wallan assembly tuned for a second wave to a second frequency lower than thefirst frequency, said network being the origin of grating lobes at thefirst frequency, and, a second network of discontinuous metal elements,said second network being likewise integral with said wall, forcompensating for said grating lobes; and wherein said dielecTric wallcomprises at least one tissue of glass fibres, the network of continuouswires being incorporated into the tissue during the weaving stage, andthe network of discontinuous metal elements being printed on adielectric substrate which is subsequently stuck to the internal face ofthe dielectric wall.
 6. A multifrequency operating radome comprising: amonolithic dielectric wall for transmitting a first wave of a firstfrequency and its harmonics; a first network of continuous wires, saidnetwork being integral with said wall, for constituting with said wallan assembly tuned for a second wave to a second frequency lower than thefirst frequency, said network being the origin of grating lobes at thefirst frequency, and, a second network of discontinuous metal elements,said second network being likewise integral with said wall, forcompensating for said grating lobes; and wherein said dielectric wallcomprises at least one tissue of glass fibres, the network of continuouswires being incorporated into the tissue during the weaving of the same,and the network of discontinuous metal elements being printed on adielectric substrate which is subsequently incorporated into thethickness of the dielectric wall.
 7. A radome as claimed in claim 4,wherein each of the discontinuous metal elements is constituted by asegment of wire of short length, compared with the shortest operatingwavelength.
 8. A radome as claimed in claim 4, wherein the network ofdiscontinuous elements and the network of continuous wires have the samepitch.
 9. A radome as claimed in claim 5, wherein each of thediscontinuous metal elements is constituted by a segment of wire ofshort length, compared with the shortest operating wavelength.
 10. Aradome as claimed in claim 5, wherein the network of discontinuouselements and the network of continuous wires have the same pitch.
 11. Aradome as claimed in claim 6, wherein each of the discontinuous metalelements is constituted by a segment of wire of short length, comparedwith the shortest operating wavelength.
 12. A radome as claimed in claim6, wherein the network of discontinuous elements and the network ofcontinuous wires have the same pitch.